1. Field of the Invention
The present invention relates to a microwave detector, and, more particularly, to a horn antenna structure therefor.
2. Brief Description of Related Art
A variety of microwave detectors, each of which is provided with a horn antenna for receiving microwave energy, have been disclosed.
For example, a structure disclosed in Japanese Patent Application Publication No. 61-500944 has been known. That is, a horn antenna element manufactured by one-piece molding is provided in such a manner that it can be mounted on a microstrip plate (called a "fastening plate" in the above-described disclosure). The above-described horn antenna element is arranged in such a manner that the antenna portion thereof, a local oscillator disposed on the microstrip plate and a shield portion for covering the mixer are integrally formed. The above-described shield portion constitutes a resonant circuit in the form of a 3-D structure and acts to prevent the external energy radiation at the local oscillating frequency.
The above-described horn antenna element has the inlet portion (which receives the microwave energy) and the bottom portion each of which is opened, the horn antenna element being made of an electroconductive material having only the top portion and the side portion.
The above-described antenna element has a shape the cross sectional area of which is gradually reduced from the inlet portion thereof.
The microstrip plate, on which the above-described horn antenna element is mounted, has a ground plate in at least a region in which said antenna and said shield portion are covered, the ground plate being formed by the same manufacturing process as that for manufacturing the microstrip or the circuit layer.
The "microstrip" is an element of a microstrip line containing component capacitor or inductor and integrally formed with the ground plate formed on the reverse side via a dielectric substrate.
The above-described horn antenna element is fixed to the microstrip plate by a screw which is inserted into the microstrip plate. At this time, the horn antennal element and the ground plate are electrically connected to each other by the above-described screw so that the ground plate serves as the bottom wall of the horn antenna element.
The local oscillator and the mixer are disposed in the shield portion of the above-described horn antenna element, the local oscillator and the mixer being included in the structure formed on the dielectric substrate made of, for example, ceramics so as to be integrally disposed.
However, according to the above-described conventional technology, the ground plate on the portion of the microstrip plate in which the horn antenna is mounted is used so as to serve as the bottom wall of the horn antenna element in order to lower the height of the horn element and to reduce the overall size of the microwave detector.
However, the realized size reduction has not been satisfactory to meet the desire to further reduce the size of the microwave detector. Therefore, there has been a desire to further lower the height of the above-described horn element.
Furthermore, another problem arises in that the number of the manufacturing process increases since the above-described local oscillator and the mixer are included on the structure of the dielectric substrate made of, for example, ceramics so as to be disposed in said horn antenna element (correctly, in the shield portion).
The reason why the above-described local oscillator and the mixer must be mounted on the microstrip plate on top of the dielectric substrate is that the ground plate is formed on the microstrip plate on the same side on which the horn antenna element is mounted and is used as the bottom wall of the above-described horn antenna element.
Furthermore, the above-described structure in which the local oscillator and the mixer are included via the dielectric substrate causes a necessity of establishing an electric connection with other circuits in the portions except for the region on which the horn antenna element is mounted. As a result, another problem arises in that the impedance matching with the above-described other circuits cannot be easily realized.